1. Field of the Invention
The present invention concerns improving the cleaning properties of organic solvents used in drycleaning operations, and specifically, concerns improving the cleaning properties of such organic drycleaning solvents by removing residual moisture therefrom as well as sulfur and other residual reducing agents by the oxidation of the same.
2. Description of the Prior Art
In commercial drycleaning operations, various types of organic solvents have been employed in the past in the drycleaning process to remove soils from fabrics and clothing. For example, the prior art has employed solvents such as halogen-substituted aliphatic hydrocarbons such as perchloroethylene (tetrachloroethylene), mineral spirits, various types of petroleum solvents, mixtures of hydrocarbon and halogen-substituted hydrocarbon solvents with detergents, and the like. Typically, the petroleum solvents employed in drycleaning operations have a boiling point in the range of from about 200.degree. F. to about 400.degree. F. and a surface tension of from about 20 to about 30 dynes/cm at room temperature (i.e., 20.degree. C.). A typical prior art formulation of a halogenated hydrocarbon solvent-detergent combination is manufactured by DuPont and sold under the trade name Valclene, which is a combination of a fluorocarbon solvent and a special high-performance detergent with a boiling point of about 118.degree. F., a density of about 13.16 pounds per gallon at 68.degree. F. and a surface tension at 68.degree. F. of about 20 dynes/cm. A suitable fluorocarbon solvent is trichlorotrifluoroethane as the solvent constituent for Valclene.
Other solvents such as carbon tetrachloride and methylisobutylketone have also been employed in the past in commercial drycleaning operations. Due to the good stability and relatively high cost of these solvents, they are naturally reused and continuously recycled in the drycleaning process, and thus it becomes desirable to recondition the solvents by removing residual moisture, sulfur, and other residual reducing agents therefrom in order to provide reconditioned solvent for cleaning successive batches of soiled fabrics and clothing.
The prior art, in an effort to remove such impurities from drycleaning solvents, suggests that the formulations used as the solvents be distilled after the drycleaning operation; however, distillation is normally expensive and sometimes difficult to conduct without costly equipment, and problems arise in some instances because of the loose additives present in the drycleaning mixtures.
Another suggested solution for the removal of the impurities from the solvents is by a filtration and absorption technique wherein the impurities are selectively removed from the solvents after the same have been used to clean soiled fabrics. However, the use of such techniques is disadvantageous in that equipment cost becomes high, and other mechanical problems with respect to the apparatus prevent this method from being effectively conducted. In addition, the loose additives which may be present in the drycleaning formulation may tend to inhibit effective filtration and absorption.
The prior art also suggests the addition of water to common formulations of drycleaning mixtures in order to improve the drycleaning characteristics of the solvents employed for the cleaning operations. Normally, water would be added to drycleaning solvents to enable the solvents to dissolve water soluble components of stains on fabrics or clothing. However, most modern fabrics are non-wettable and the presence of water, even in small amounts, prevents the organic solvents, which are generally non-polar in nature, from dissolving slightly polar stains, such as fats, etc., in the solvents since the stains become hydrated through dipole-dipole bonding with the water molecules.
During the drycleaning operation, the relative humidity of the solvent changes according to the humidity of the ambient air and that present in the fabric or clothing being cleaned. Normally, the relative humidity of the solvents is maintained at about 65-70% in order to dissolve, for example, salts and sugars from stains. However, the control of the amount of moisture which is present in organic drycleaning solvents is very important since serious problems are encountered if the amount of moisture in the solvents becomes either too high or too low.
Too much moisture in drycleaning solvents may result in fiber shrinkage or distortion, the dulling of colors of the fabrics or clothing being cleaned and possible fading of the fabrics. In addition, fabrics become harder to press following cleaning. Too low a moisture level in the solvent results in the redepositing of water-soluble substances on the fabrics or clothing being cleaned, with the result that stains cannot be effectively removed. Further, a low moisture content in the drycleaning solvent may result in carbon being redeposited on the fabric, thereby adversely affecting the color of the fabric and leaving the fabric with static electricity charges which make it uncomfortable for a person to subsequently wear the clothing; in addition, the presence of static electricity charges present a danger of explosion with certain types of solvents.
In addition to the problems inherent in the use of drycleaning solvents containing too much or too little moisture, other problems are apparent due to the presence of impurities, such as sulfur and other reducing agents in drycleaning solvents. Such impurities may deposit on the fibers of the clothing being cleaned, resulting in discoloration and spotting thereof, or their presence in the solvents may result in fading of the colors of the fabrics being cleaned, may cause odors to remain on the cleaned products, and in addition, especially with respect to sulfur impurities, may cause corrosion of the equipment being used to clean the material.
Accordingly, it would be desirable to very strictly control the moisture content of drycleaning solvents in order to prevent, primarily, shrinkage of clothing being cleaned, and also to prevent adverse static electricity effects. Further, it would be desirable to remove impurities such as sulfur from drycleaning solvents in order to make the drycleaning process more effective and to prevent adverse effects such as color fading and corrosion of the equipment employed in the drycleaning operation.
In the inventor's U.S. Pat. No. 3,766,075, an invention is disclosed whereby residual moisture and impurities contained in typical formulations of drycleaning solvents are removed, without the adverse effects known to the prior art, and adding an unsubstituted aromatic hydrocarbon or a halogen- or alkyl-substituted aromatic hydrocarbon together with a dried cellulosic material onto which has been precipitated a fine deposit of an oxidizing agent such as lead dichromate to an organic drycleaning solvent. The treated cellulosic material is disclosed in that patent as absorbing excess moisture present in the solvent mixture as well as removing impurities therefrom through oxidation by means of the employed dichromate. In the inventor's copending U.S. application Ser. No. 657,332, filed Feb. 11, 1976, now issued as U.S. Pat. No. 4,077,878, an improvement in the aforesaid process is disclosed wherein the dichromate compound is precipitated onto a dried cellulosic material which is contained in a permeable cellulosic bag. The bag is electrically grounded by attaching a wire thereto or the bag is attached to a ground via a DC power supply.
The present invention is an improvement on the preceding drycleaning method.
According, it is the principal object of the present invention to provide a means for removing residual moisture from typical formulations of drycleaning solvents.
It is a further object of the present invention to provide improved drycleaning compositions free of residual moisture and impurities, without the adverse effects known in the prior art.
The present invention provides an improved drycleaning system and method which comprises adding an unsubstituted or halogen- or alkyl-substituted aromatic hydrocarbon to an organic drycleaning solvent and circulating the mixture to a filter arrangement comprising a first filter wherein the mixture is contacted with a solid acid, and a second filter wherein the mixture is contacted with chromate compound. The chromate compound or, in the case where the filter containing the chromate compound is metallic, the filter itself is grounded or attached to a ground via a DC power supply.